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I have two cupresocyparises (aka leylandiis) that each developed two leaders. They are both around 8m (25ft) high now, the leaders start at appr. 2m above ground, so I guess it is too late just to remove one of the leaders in its entirety. The distance between leaders is small - 10 cm, and each leader 'has' its own half of the tree (above the starting point of leaders of course), but those two halves are knitted closely, impossible to move separately.

This is not my photo, but in the center right of following photo there is a tree with double leader somewhat similar to mine (but in an early stage of development)

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This is one of mine double-leader Leylandiis:

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From aesthetics point of view, they look fine to me.

What is the significance of double leaders regarding other aspects of health and development?

What if I just leave them?

Can I just shorten one by lets say 40cm, hoping that the remaining will become the only leader over time?

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Leyland Cypress are able to form multiple leaders but we always chose one and suppressed the other or others easily just by cutting the leaders completely off from the main branch or trunk or where it branched from the leader you want to keep. We did this when we planted them. When they were easy to prune lying on the ground. If multiple leaders formed later we left them alone. Mostly we hedged these trees and 15 feet was as high as we'd allow the hedges to grow.

Since you are allowing them to become the form they are genetically predisposed to have cutting off one of the leaders will redirect all that wasted energy from the errant leader to go back to the rest of the tree.

The apical tip of a leader has more energy than any of the other growth tips. I think it is a waste of energy to have rivaling leaders. The other problem is there is usually a weaker, narrow angle between the two leaders. When they get older and that Y gets bigger with heavier branches then you will have a problem where that Y will break and the branch's weight will rip the Y and possibly the entire junction killing both leaders.

Where ever those two leaders branch at the trunk or where they divided from each other...that Y will always stay at that level. That height. It is interesting to note that all branches coming from the trunk that that Y of branch and trunk will always stay at that level. In fact that spot will lower as the diameter of the branch grows larger.

If you cut one leader back, say a foot or more leaving a few feet or more of that leader below, the energy from that tip will be transferred down just that branch giving the lateral buds more energy in just that branch. It could produce a too heavy branch when all the lateral buds start growing and you get a 'witches broom'. That will not look good and will then be made susceptible to breaking off earlier and will be obvious.

The leaders will continue to grow taller and thicker. Take a picture that shows where the two leaders begin and we can help you decide which leader to keep and where to cut the other.

Now, if you do not have a ladder that tall, you could simply leave those leaders alone as Bamboo suggests. It will be years before any problems will arise if at all. When they get bigger you'll be having branches breaking now and then anyway. No worry about buildings or fences or people to fall on? I'd leave them alone myself.

Your trees look great. Plenty of room to just let them grow into their natural form. Very decently made wind break and sound barrier!

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  • Two leaders are almost identical, meaning height, diameter at Y point, number of branches etc. Like twins. – VividD Dec 1 '17 at 8:39
  • The leaders go straight upward after Y point, so this is more U point rather than Y. – VividD Dec 1 '17 at 8:57
  • Well that would be a better joint than the Y...you could tip one of the leaders down about a foot or two just to see what happens. If you can get up there now, you'll be able to get up there later if the witch's broom thing happens. Or do nothing. – stormy Dec 1 '17 at 20:59
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Leave them as they are - Leylandii often have more than one apically dominant shoot, and attempting to reduce the number of them may encourage even more to form.

Generally, Leylandii are used as hedging and, as such, will be pruned back yearly or bi yearly anyway, but even as free standing specimen trees, at a height of 8 metres which you say yours have already reached, I wouldn't recommend trying to get a single apically dominant shoot because it'll effectively create an unnecessary wound, particularly as you do not find this an unattractive look.

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